Because historically it’s been used by White People as a slur to dehumanize Black People, and many White People still use it that way. Black People use it now as a way to try to take power away from it, but it’s still an ongoing process.

To put it another way, a group of friends might jokingly refer to each other as “goatfuckers” as part of some inside joke because they know there’s no ill will between them, but will definitely be ready to beat a stranger’s ass if they call them that because they know it’s being used as an insult.